Due to their good ratio of rigidity or strength to density, composite materials, and in particular sandwich constructions, have a wide range of applications in the field of aircraft construction. Generally speaking, sandwich constructions are made from a top and a bottom cover layer, between which, for the purpose of enhanced rigidity, there can be a honeycomb-like core structure, for example made of vertically extending cells of hexagonal cross section.
As an alternative to the design involving honeycomb structures, rigid cellular materials can be used. However, sandwich constructions comprising a rigid cellular material core are to some extent associated with a disadvantage in that, when compared to sandwich constructions with a honeycomb core structure and comparable density, their mechanical characteristics are inferior. In order to compensate for this, fibers, threads or protruded semi-finished frame products can be incorporated in the rigid cellular material at defined angles and at a defined density. In the case of fibers or threads and a subsequent resin infiltration process, the fibers then contribute to the mechanical reinforcement of the cellular material. In this case the cellular material not only acts as a carrier that holds the pins in the form of the resin-reinforced fibers or threads in position, but also serves to stabilize the pins in order to prevent or at least delay any buckling or collapsing of said pins when under load.
However, since the load-bearing capacity of such reinforced rigid cellular materials is decisively determined by the introduced pins or by introduced protruded semi-finished frame products, as a rule the existing cellular core in an undesirable manner tends to contribute to an increase in the density of the core structure. Furthermore, as a rule, a strengthened cellular material structure comprises only a small region in which it is elastic under load so that as a rule damage to the composite material tends to be plastic and permanent. Lastly, aeration or dewatering of a sandwich structure with a reinforced rigid cellular material is not possible because the space between the cover layers is completely filled by the rigid cellular material.
From WO 2004/022869 A2 and WO 03/101721 A1, for example, methods for manufacturing a three dimensional lattice structure are known, in which methods at first metallic lattice structures are generated which by means of a bottom die and an associated upper die are bent to the third dimension so that a three dimensional latticework is created. During such bending, the lateral border of the metallic lattice mat is not held in place because this would prevent any bending to the third dimension. However, such bending with the use of a bottom die and associated top die is comparatively inflexible, because varying the latticework angle and varying the height of the latticework requires a change of the bottom die and of the associated upper die.
U.S. Pat. No. 3,884,646 also describes a manufacturing process for a three dimensional latticework for use in a sandwich construction as a core structure. In this method, first a flat lattice structure is formed from a metal sheet, which lattice structure is subsequently bent, again by means of a forming process, by means of a bottom die and associated upper die, in order to impart a three dimensional shape to said flat lattice structure.
While the three dimensional lattice structures manufactured according to the above-mentioned printed publications are not associated with the disadvantages of cellular-material-reinforced core structures as explained above, the manufacturing methods, as explained, for manufacturing three dimensional lattice structures are comparatively inflexible due to the use of a bottom die and upper die.
Among other things there may be a need to state a method for manufacturing a three dimensional frame structure without the use of a carrier material, for example in the form of a rigid cellular material, wherein the frame structure in relation to producing various lattice geometries is more flexible than the described forming methods using a bottom die and upper die. In addition, other objects, needs, desirable features, and characteristics will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and this background.